In the typical operation of electrothermal phosphorus furnaces, the gaseous products, consisting mainly of phosphorus vapour, carbon monoxide gas and entrained amounts of dust made up of fine particles of the raw material burden of the furnace, are removed from the top of the furnace, and are passed through an electrostatic precipitator kept at a temperature above the dew point of the phosphorus to remove the dust from the gaseous products stream. The phosphorus vapour and carbon monoxide gas leaving the precipitator, desirably free of particulates, is forwarded to a series of water spray condensers, wherein the phosphorus vapour is liquified and collected in sumps under the condensers. The remaining carbon monoxide gas is usually burned in a flambeau.
Efficient functioning of the electrostatic precipitator is highly desirable since any dust which is not removed from the gaseous products stream therein accumulates in the condenser sumps, causing the formation of an undesirable phosphorus-water-solid particulate emulsion, commonly known as mud. In actual practice, the precipitators are not efficient in dust removal and phosphorus mud formation occurs. Typical analysis of the mud is about 30% by weight phosphorus, about 55% by weight water and about 15% by weight solids. Since the recovery of phosphorus from mud is difficult and expensive, the formation of mud in the phosphorus furnace process constitutes a nuisance, representing considerable handling and disposal problems and often an economic loss of phosphorus values.
The size and construction of the electrostatic precipitators is such that the development of leaks in the body of the precipitator is practically impossible to avoid during operation. For this reason the precipitator is typically operated under vacuum so that gases do not enter the outside atmosphere around the unit and cause hazardous working conditions. However, operation under vacuum results in the ingress of some air into the body of the precipitator, causing combustion of some of the phosphorus vapour and consequently the formation of phosphoric acid and also some amorphous phosphorus. These materials, when mixed with dust withdrawn from the precipitator, introduce additional handling difficulties with respect to the precipitator dust and represent a loss of phosphorus. Some of the acid formed in this manner passes with the gaseous products stream to the condensers, ending up in the condenser sumps, and has to be neutralized there with an alkali, such as, ammonia or sodium hydroxide.
Since the temperature inside the precipitator has to be maintained above the dew point of phosphorus in order to avoid condensation of liquid phosphorus inside the unit, the gases leaving the furnace are typically kept at temperatures in excess of 300.degree. C., up to, typically, about 450.degree. C. Such high temperature operation is deleterious to the equipment and contributes to process control difficulties. When, from time to time, due to temperature fluctuations caused by erratic control, there occurs condensation of phosphorus inside the precipitator, the condensed phosphorus in the dust withdrawn from the unit will fire up and burn, causing very difficult working conditions and further loss of phosphorus.
Thus, the handling of the gaseous products of phosphorus furnaces using electrostatic precipitators, as practised by the prior art, presents numerous problems and is not generally satisfactory. While precipitators are considered an essential part of the furnace gas handling system to minimize mud formation, they are notoriously inefficient, difficult to control and are one of the major contributors to poor working conditions in phosphorus furnace plants. The capital cost of precipitators is very high and the cost of their maintenance is considerable.
There have been various attempts in the past to bypass the electrostatic precipitators by connecting the furnace gas off-take directly to the condensers. These attempts have, however, generally resulted in a considerably increased generation of mud which has been considered unacceptable, in view of the problems associated with phosphorus recovery from the mud, as discussed above.